Larval supply and recruitment of the ivory barnacle Balanus eburneus (Gould, 1841) on oyster cultch used in oyster-reef restoration

Larval supply controls larval recruitment on a regional scale. Near to the substratum interface, however, larval preferences and boundary flow may be overriding influences. Restored oyster reefs are intended to provide substrata for sessile hard-fouling organisms such as barnacles, but the optimal configuration to encourage larval settlement has not yet been determined. Two common restoration strategies involve cultch, the shell material for reef restoration, in bags or on mats. Bags constitute quick, stackable construction with cultch packed haphazardly and limited space between shells. Mats utilize cultch sparingly and keep shells more exposed to currents. Although both restoration strategies are widely used, data comparing larval settlement between bags and mats are unavailable. We followed daily larval supply of the ivory barnacle Balanus eburneus (Gould, 1841) and corresponding settlement on cultch presented in bags and mats in a field study lasting one month in an estuary in eastern Florida, USA. Consideration was also given to the effect of substratum orientation (upward-facing, downward-facing, or vertical). Recruitment densities of B. eburneus were as great as 7.6 ± 0.54 SE individuals cm–2, and higher on mats than in bags (P = 0.0001). Orientation of the substratum also influenced recruitment, with greater settlement on downward-facing and vertical oyster shells presented on mats (P = 0.037), which were more open to boundary flow. Peak larval densities were 5.2 ± 0.8 SE individuals liter–1. Larval supply correlated best with recruitment 8 days later (R2 = 0.71; P = 0.001), but recruitment rates were also influenced by the tertiary configuration and orientation of cultch substrata. If the goals of oyster reef restoration include creating a structure conducive to the settlement of sessile hard-fouling organisms such as barnacles, we recommend the use of oyster mats with shells secured in a downward-facing or vertical position during mat assembly.